Some of conventional optical heads for optical disks are provided with a function of detecting a relative tilt of optical axes of the optical disk and the optical head as disclosed in Japanese Unexamined Patent Publication No. 2003-45058.
FIG. 22 is a diagram showing the construction of the conventional optical head disclosed in the above publication. Identified by 101 is a light source, by 102 an optical axis of the optical head, by 103 a beam splitter, by 104 an objective lens, by 105 an optical disk, by 106 a detecting optical system, by 107 a light sensing means, by 108 a tilt detecting means for detecting the tilt of the optical disk, by 109 a signal calculating section, by 110 an amplifier for amplifying an input signal by a factor of k0, by 111 a differential amplifier, by PP1, PP2 two difference signals detected by the signal  calculating section 109, and by TILT a tilt detecting signal generated by the tilt detecting means 108.
FIG. 23 is a diagram showing a light sensing area of the light sensing means 107 and a luminous flux incident thereon. As shown in FIG. 23, the light sensing area 107 is comprised of six regions 107a to 107f for detecting luminous energies corresponding to a luminous flux 112 incident thereon. Two regions 112a, 112b encircled by arcs at the left and right sides of the luminous flux 112 represent overlapping regions of 0th-order components and ±1st-order components of the light diffracted by grooves of information tracks of the optical disk 105. Arrows in FIG. 23 show directions tangent to the information tracks.
As shown in FIG. 22, a laser beam emitted from the light source 101 passes through the beam splitter 103, and is gathered onto an information recording surface of the optical disk 105 by the objective lens 104. The beam reflected by the optical disk 105 passes through the objective lens 104 again and is reflected by the beam splitter 103. This beam is introduced to the light sensing means 107 by the detecting optical system 106.
As shown in FIG. 23, the luminous flux 112 incident on the light sensing means 107 is sensed in the respective light sensing regions 107a to 107f, and the difference signals PP1 and PP2 are detected by the signal calculating section 109. These two difference signals PP1, PP2 can be expressed as follows  using electrical signals outputted from the respective light sensing regions 107a to 107f:PP1=107c+107e−(107d+107f)PP2=107a−107b.It should be noted that 107a to 107f in the above equations mean the electrical signals outputted from the respective light sensing regions.
In the tilt detecting means 108, after being amplified by a factor of k0 in the amplifier 110, the difference signal PP1 is subtracted from the difference signal PP2 by the differential amplifier 111, with the result that the signal TILT is outputted. Specifically, the signal TILT is expressed as follows:TILT=PP2−k0*PP1 (where * denotes multiplication throughout the specification).Here, a factor k0 is so determined as to correct an offset of the difference signal PP2 caused by a relative displacement of the optical axis of the objective lens 104 and the optical axis 102 of the optical head using an offset produced in the difference signal PP1. Thus, the signal TILT is a signal free from an offset caused by the displacement of the objective lens 104.
In the case that the optical disk 105 is inclined with respect to the optical axis 102 of the optical head, a coma aberration occurs when the light passes through a transparent  substrate of the optical disk 105. This coma aberration mainly deforms the wavefront of the sections where the aforementioned 0th-order components and ±1st-order components of the diffracted light from the information track overlap. The deformation of the wavefront differs in the regions for detecting the difference signal PP1 and in the region for detecting the difference signal PP2, and the signals detected in these regions are differently modulated by the information tracks. Thus, this difference in modulation represents the tilt of the optical disk and appears in the signal TILT. Accordingly, the detection of the tilt of the optical disk unlikely influenced by the displacement of the objective lens 104 can be made possible by detecting the signal TILT while a light spot is tracing the center of the information tracks.
However, with the construction of the aforementioned conventional optical head, reflectivity differs at the information tracks having information recorded thereon and at those having no information recorded thereon. For example, in an optical disk of the phase changing type or the like, the symmetry of a light intensity distribution in a middle part of the luminous flux largely changes, thereby presenting a problem that an optical-disk tilt detecting signal has a detection error.
It is known that the influence of the change in the symmetry of the difference signals PP1 and PP2 can be reduced by  arranging a light blocking portion 113 (N region) in a region (region containing only or mainly the 0th-order components of the diffracted light) in the middle part of the luminous flux 112 where the symmetry of the light intensity distribution largely changes as shown in FIG. 24.
FIG. 25 diagrammatically shows the section of the information tracks of the optical disk 105. The respective information tracks are affixed with information track numbers 1 to 9.
Out of the information tracks 1 to 9, the tracks 4 to 6 are those having information recorded thereon and the tracks 1 to 3, 7 to 9 are those having no information recorded thereon. FIG. 25 shows that recording is made only to the hatched information tracks 4 to 6 to thereby reduce the reflectivity.
FIGS. 26A, 26B show a simulation result of the level of the signal TILT produced when a light spot crosses these information tracks 1 to 9, assuming that the pattern of the information tracks shown in FIG. 25 are cyclically repeated.
Calculation conditions were as follows. Specifically, the wavelength of the light source was 405 mm; NA of the objective lens 0.85; the thickness of the transparent substrate of the optical disk 100 μm; the pitches of the information tracks 0.32 μm; the width of the grooves of the information tracks 0.2 μm; the depth of the information tracks 1/12 of the wavelength; the reflectivity of the recorded information tracks 0.6; the  reflectivity of the non-recorded information tracks 1.0; and the tilt of the optical disk 0 deg. Further, the width of the light sensing regions 107a, 107b for detecting the signal PP2 along the direction of the information tracks was 0.3 times the diameter of the luminous flux, that of the light sensing regions 107c, 107d, 107e, 107f for detecting the signal PP1 along the same direction was 0.6 times the diameter of the luminous flux, and the outside region was not considered as the calculation condition. Further, the width of the light blocking portion 113 (N region) along a direction normal to the information tracks was 0.35 times the diameter of the luminous flux, and a value of the factor k0 was so determined as to correct an offset produced when the aforementioned displacement of the objective lens is ±100 μm and was 1.20 this time.
The calculation results are shown in FIGS. 26A, 26B. Three sequential lines of FIG. 26A were calculated when the displacement of the objective lens was 0 μm and ±100 μm with a defocusing amount fixed at 0 μm. Five sequential lines of FIG. 26B were calculated when the defocusing amount was 0 μm, ±0.1 μm and ±0.2 μm with the displacement of the objective lens fixed at 0 μm. In FIGS. 26A, 26B, horizontal axis corresponds to the information track numbers of FIG. 25 and vertical axis represents an optical-disk tilted amount (unit: deg) converted from the optical-disk tilt detecting signal TILT. 
The following can be understood from FIGS. 26A and 26B. Specifically, in the case of determining the factor k0 to suppress a variation of the optical-disk tilt detecting signal caused by the displacement of the objective lens, the optical-disk tilt detecting signal, i.e. the tilt detecting signal can be suppressed to a sufficiently small level regardless of the track number even if the optical axis of the objective lens is displaced from that of the optical head as shown in FIG. 26A. On the other hand, in the case of defocusing, the level of the optical-disk tilt detecting signal changes particularly near the tracks 3, 4, 6, 7 as boundaries between the recorded information tracks and the non-recorded information tracks as shown in FIG. 26B. Therefore, an error is produced at the boundaries between the recorded information tracks and the non-recorded information tracks as if the tilt of the optical disk were detected.
In view of the problems residing in the prior art, an object of the present invention is to provide an optical head, an integrated circuit and an optical disk device capable of obtaining an optical-disk tilt detecting signal having a little influence of defocusing even at boundaries between recorded information tracks and non-recorded information tracks.
To accomplish the above object, an aspect of the invention is directed to an optical head, comprising a light source, an objective lens for concentrating a laser beam emitted from the light source onto information tracks of an optical disk, and a  light sensing device for sensing a luminous flux reflected by information tracks, wherein: the light sensing device includes a luminous flux incident area divided into a plurality of regions by a straight vertical dividing line passing an optical axis of the objective lens and parallel with a direction tangent to the information tracks, straight first and second horizontal dividing lines normal to the vertical dividing line and symmetrically arranged with respect to the optical axis, a first and a second partition lines symmetrically arranged between the two horizontal dividing lines with respect to the vertical dividing line while being distanced from the vertical dividing line and the two horizontal dividing lines; each of the first and second partition lines includes a pair of horizontal lines parallel with the two horizontal dividing lines, and an inner line connecting ends of the horizontal lines toward the vertical dividing line; the luminous flux incident area further includes an N region crossing over the vertical dividing line between the two horizontal dividing lines, one region partitioned between the two horizontal dividing lines by the vertical dividing line being divided into an A2 region at a side toward the optical axis and a remaining A1 region by the first partition line, and the other region partitioned between the two horizontal dividing lines by the vertical dividing line being divided into a B2 region at a side toward the optical axis and a remaining B1 region by the second partition line; the A1 region and the B1  region are symmetrically arranged with respect to the vertical dividing line; the A2 region and the B2 region are symmetrically arranged with respect to the vertical dividing line; and the optical head further comprises a tilt detector for detecting a relative tilt of the objective lens and the optical disk in accordance with a luminous energy incident on the luminous flux incident area of the light sensing device.
Another aspect of the invention is directed to an optical head, comprising a light source, an objective lens for concentrating a laser beam emitted from the light source onto information tracks of an optical disk, and a light sensing device for sensing a luminous flux reflected by information tracks, wherein: the light sensing device includes a luminous flux incident area divided into a plurality of regions by a straight vertical dividing line passing an optical axis of the objective lens and parallel with a direction tangent to the information tracks, a first and a second partition lines symmetrically arranged with respect to the vertical dividing line while being distanced from the vertical dividing line, a third partition line arranged between the vertical dividing line and the first partition line, a fourth partition line symmetrically arranged with the third partition line with respect to the optical axis between the vertical dividing line and the second partition line; each of the first and second partition lines includes a pair of horizontal lines normal to  the vertical dividing line, and an inner line connecting ends of the horizontal lines toward the vertical dividing line; each of the third and fourth partition lines includes a pair of horizontal lines normal to the vertical dividing line, and an inner line connecting ends of the horizontal lines toward the vertical dividing line; the luminous flux incident area further includes an N region crossing over the vertical dividing line, a region at a side of the optical axis opposite from the third partition line being divided into an A5 region at a side toward the third partition line and a remaining A1 region by the first partition line, and a region at a side of the optical axis opposite from the fourth partition line being divided into a B5 region at a side toward the fourth partition line and a remaining B1 region by the second partition line; the A1 region and the B1 region are symmetrically arranged with respect to the vertical dividing line; the A5 region and the B5 region are symmetrically arranged with respect to the vertical dividing line; and the optical head further comprises a tilt detector for detecting a relative tilt of the objective lens and the optical disk in accordance with a luminous energy incident on the luminous flux incident area of the light sensing device.
A further aspect of the invention is directed to an optical head, comprising a light source, an objective lens for concentrating a laser beam emitted from the light source onto information tracks of an optical disk, and a light sensing  device for sensing a luminous flux reflected by information tracks, wherein: the light sensing device includes a luminous flux incident area divided into a plurality of regions by a straight vertical dividing line passing an optical axis of the objective lens and parallel with a direction tangent to the information tracks, straight first and second horizontal dividing lines normal to the vertical dividing line and symmetrically arranged with respect to the optical axis, a third and a fourth horizontal dividing lines at a distance to and parallel with the horizontal dividing lines in one region partitioned by the vertical dividing line between the first and second horizontal dividing lines, a fifth and a sixth horizontal dividing lines at a distance to and parallel with the horizontal dividing lines in the other region partitioned by the vertical dividing line, a seventh and an eighth horizontal dividing lines arranged at sides of the first and second horizontal dividing lines opposite from the optical axis, in parallel with the first and second horizontal dividing lines and symmetrically with respect to the optical axis; the luminous flux incident area further includes an N region crossing over the vertical dividing line between the first and second horizontal dividing lines; a region between the first and second horizontal dividing lines in the one region partitioned by the vertical dividing line is divided into an A1 region between the third and fourth horizontal dividing lines and remaining A7 regions; a region  between the first and second horizontal dividing lines in the other region partitioned by the vertical dividing line is divided into a B1 region between the fifth and sixth horizontal dividing lines and remaining B7 regions; regions at sides of the seventh and eighth horizontal dividing lines opposite from the optical axis are defined as N2 regions; the A1 region and the B1 region are symmetrically arranged with respect to the vertical dividing line; the A7 region and the B7 region are symmetrically arranged with respect to the vertical dividing line; and the optical head further comprises a tilt detector for detecting a relative tilt of the objective lens and the optical disk in accordance with a luminous energy incident on the luminous flux incident area of the light sensing device.
A further aspect of the invention is directed to an optical head, comprising a light source, an objective lens for concentrating a laser beam emitted from the light source onto information tracks of an optical disk, and a light sensing device for sensing a luminous flux reflected by information tracks, wherein: the light sensing device includes a luminous flux incident area divided into a plurality of regions by a straight vertical dividing line passing an optical axis of the objective lens and parallel with a direction tangent to the information tracks; a pair of first horizontal dividing lines normal to the vertical dividing line and symmetrically arranged with respect to the vertical dividing line; a pair of second  horizontal dividing lines parallel with the pair of first horizontal dividing lines and symmetrically arranged with respect to the vertical dividing line; a third and a fourth horizontal dividing lines at a distance to and parallel with the first and second horizontal dividing lines in one region partitioned by the vertical dividing line between the first and second horizontal dividing lines; a fifth and a sixth horizontal dividing lines at a distance to and parallel with the first and second horizontal dividing lines in the other region partitioned by the vertical dividing line between the first and second horizontal dividing lines; a first inner line extending in parallel with the vertical dividing line and connecting inner ends of the first horizontal dividing line at one side, the third horizontal dividing line, the fourth horizontal dividing line and the second horizontal dividing line at one side; a second inner line extending in parallel with the vertical dividing line and connecting inner ends the first horizontal dividing line at the other side, the fifth horizontal dividing line, the sixth horizontal dividing line and the second horizontal dividing line at the other side; and seventh and eighth horizontal dividing lines arranged at sides of the first and second horizontal dividing lines opposite from the optical axis, in parallel with the first and second horizontal dividing lines and symmetrically with respect to the optical axis: the luminous flux incident area further includes an N region  crossing over the vertical dividing line between the first and second inner lines: a region defined by the first horizontal dividing line, the second horizontal dividing line and the first inner line in the one region partitioned by the vertical dividing line is divided into an A1 region between the third and fourth horizontal dividing lines and remaining A7 regions: a region defined by the first horizontal dividing line, the second horizontal dividing line and the second inner line in the other region partitioned by the vertical dividing line is divided into a B1 region between the fifth and sixth horizontal dividing lines and remaining B7 regions: regions at sides of the seventh and eighth horizontal dividing lines opposite from the optical axis are defined as N2 regions: the pair of first horizontal dividing lines and the pair of second horizontal dividing lines are symmetrically arranged with respect to the optical axis: the A1 region and the B1 region are symmetrically arranged with respect to the vertical dividing line: the A7 region and the B7 region are symmetrically arranged with respect to the vertical dividing line: and the optical head further comprises a tilt detector for detecting a relative tilt of the objective lens and the optical disk in accordance with a luminous energy incident on the luminous flux incident area of the light sensing device.
A further aspect of the invention is directed to an optical disk device, comprising: an optical head including a light source, an objective lens for concentrating a laser beam emitted  from the light source onto information tracks of an optical disk, and a light sensing device for sensing a luminous flux reflected by information tracks; an optical disk driver for driving the optical disk; and a controller for controlling the optical head and the optical disk driver, wherein: the light sensing device includes a luminous flux incident area divided into a plurality of regions by a straight vertical dividing line passing an optical axis of the objective lens and parallel with a direction tangent to the information tracks, straight first and second horizontal dividing lines normal to the vertical dividing line and symmetrically arranged with respect to the optical axis, a first and a second partition lines symmetrically arranged between the two horizontal dividing lines with respect to the vertical dividing line while being distanced from the vertical dividing line and the two horizontal dividing lines; each of the first and second partition lines includes a pair of horizontal lines parallel with the two horizontal dividing lines, and an inner line connecting ends of the horizontal lines toward the vertical dividing line; the luminous flux incident area further includes an N region crossing over the vertical dividing line between the two horizontal dividing lines, one region partitioned between the two horizontal dividing lines by the vertical dividing line being divided into an A2 region at a side toward the optical axis and a remaining A1 region by the first partition line, and the other region partitioned between  the two horizontal dividing lines by the vertical dividing line being divided into a B2 region at a side toward the optical axis and a remaining B1 region by the second partition line; the A1 region and the B1 region are symmetrically arranged with respect to the vertical dividing line; the A2 region and the B2 region are symmetrically arranged with respect to the vertical dividing line; and the optical head further comprises a tilt detector for detecting a relative tilt of the objective lens and the optical disk in accordance with a luminous energy incident on the luminous flux incident area of the light sensing device.
A further aspect of the invention is directed to an optical disk device, comprising: an optical head including a light source, an objective lens for concentrating a laser beam emitted from the light source onto information tracks of an optical disk, and a light sensing device for sensing a luminous flux reflected by information tracks; an optical disk driver for driving the optical disk; and a controller for controlling the optical head and the optical disk driver, wherein: the light sensing device includes a luminous flux incident area divided into a plurality of regions by a straight vertical dividing line passing an optical axis of the objective lens and parallel with a direction tangent to the information tracks, a first and a second partition lines symmetrically arranged with respect to the vertical dividing line while being distanced from the vertical dividing line, a third partition line arranged between  the vertical dividing line and the first partition line, a fourth partition line symmetrically arranged with the third partition line with respect to the optical axis between the vertical dividing line and the second partition line; each of the first and second partition lines includes a pair of horizontal lines normal to the vertical dividing line, and an inner line connecting ends of the horizontal lines toward the vertical dividing line; each of the third and fourth partition lines includes a pair of horizontal lines normal to the vertical dividing line, and an inner line connecting ends of the horizontal lines toward the vertical dividing line; the luminous flux incident area further includes an N region crossing over the vertical dividing line, a region at a side of the third partition line opposite from the optical axis being divided into an A5 region at a side toward the third partition line and a remaining A1 region by the first partition line, and a region at a side of the fourth partition line opposite from the optical axis being divided into a B5 region at a side toward the fourth partition line and a remaining B1 region by the second partition line; the A1 region and the B1 region are symmetrically arranged with respect to the vertical dividing line; the A5 region and the B5 region are symmetrically arranged with respect to the vertical dividing line; and the optical head further comprises a tilt detector for detecting a relative tilt of the objective lens and the optical disk in accordance with a luminous energy  incident on the luminous flux incident area of the light sensing device.
A further aspect of the invention is directed to an optical disk device, comprising: an optical head including a light source, an objective lens for concentrating a laser beam emitted from the light source onto information tracks of an optical disk, and a light sensing device for sensing a luminous flux reflected by information tracks; an optical disk driver for driving the optical disk; and a controller for controlling the optical head and the optical disk driver, wherein: the light sensing device includes a luminous flux incident area divided into a plurality of regions by a straight vertical dividing line passing an optical axis of the objective lens and parallel with a direction tangent to the information tracks, straight first and second horizontal dividing lines normal to the vertical dividing line and symmetrically arranged with respect to the optical axis, a third and a fourth horizontal dividing lines at a distance to and parallel with the horizontal dividing lines in one region partitioned by the vertical dividing line between the first and second horizontal dividing lines, a fifth and a sixth horizontal dividing lines at a distance to and parallel with the horizontal dividing lines in the other region partitioned by the vertical dividing line between the first and second horizontal dividing lines, a seventh and an eighth horizontal dividing lines arranged at sides of the first and second horizontal  dividing lines opposite from the optical axis, in parallel with the first and second horizontal dividing lines and symmetrically with respect to the optical axis; the luminous flux incident area further includes an N region crossing over the vertical dividing line between the first and second horizontal dividing lines; a region between the first and second horizontal dividing lines in the one region partitioned by the vertical dividing line is divided into an A1 region between the third and fourth horizontal dividing lines and remaining A7 regions; a region between the first and second horizontal dividing lines in the other region partitioned by the vertical dividing line is divided into a B1 region between the fifth and sixth horizontal dividing lines and remaining B7 regions; regions at sides of the seventh and eighth horizontal dividing lines opposite from the optical axis are defined as N2 regions; the A1 region and the B1 region are symmetrically arranged with respect to the vertical dividing line; the A7 region and the B7 region are symmetrically arranged with respect to the vertical dividing line; and the optical head further comprises a tilt detector for detecting a relative tilt of the objective lens and the optical disk in accordance with a luminous energy incident on the luminous flux incident area of the light sensing device.
A further aspect of the invention is directed to an optical disk device, comprising: an optical head including a light source, an objective lens for concentrating a laser beam emitted  from the light source onto information tracks of an optical disk, and a light sensing device for sensing a luminous flux reflected by information tracks; an optical disk driver for driving the optical disk; and a controller for controlling the optical head and the optical disk driver, wherein: the light sensing device includes a luminous flux incident area divided into a plurality of regions by a straight vertical dividing line passing an optical axis of the objective lens and parallel with a direction tangent to the information tracks; a pair of first horizontal dividing lines normal to the vertical dividing line and symmetrically arranged with respect to the vertical dividing line; a pair of second horizontal dividing lines parallel with the pair of first horizontal dividing lines and symmetrically arranged with respect to the vertical dividing line; a third and a fourth horizontal dividing lines at a distance to and parallel with the first and second horizontal dividing lines in one region partitioned by the vertical dividing line between the first and second horizontal dividing lines; a fifth and a sixth horizontal dividing lines at a distance to and parallel with the first and second horizontal dividing lines in the other region partitioned by the vertical dividing line between the first and second horizontal dividing lines; a first inner line extending in parallel with the vertical dividing line and connecting inner ends of the first horizontal dividing line at one side, the third horizontal dividing line, the fourth horizontal dividing  line and the second horizontal dividing line at one side; a second inner line extending in parallel with the vertical dividing line and connecting inner ends the first horizontal dividing line at the other side, the fifth horizontal dividing line, the sixth horizontal dividing line and the second horizontal dividing line at the other side; and seventh and eighth horizontal dividing lines arranged at sides of the first and second horizontal dividing lines opposite from the optical axis, in parallel with the first and second horizontal dividing lines and symmetrically with respect to the optical axis: the luminous flux incident area further includes an N region crossing over the vertical dividing line between the first and second inner lines: a region defined by the first horizontal dividing line, the second horizontal dividing line and the first inner line in the one region partitioned by the vertical dividing line is divided into an A1 region between the third and fourth horizontal dividing lines and remaining A7 regions: a region defined by the first horizontal dividing line, the second horizontal dividing line and the second inner line in the other region partitioned by the vertical dividing line is divided into a B1 region between the fifth and sixth horizontal dividing lines and remaining B7 regions: regions at sides of the seventh and eighth horizontal dividing lines opposite from the optical axis are defined as N2 regions: the pair of first horizontal dividing lines and the pair of second horizontal dividing lines  are symmetrically arranged with respect to the optical axis: the A1 region and the B1 region are symmetrically arranged with respect to the vertical dividing line: the A7 region and the B7 region are symmetrically arranged with respect to the vertical dividing line: and the optical head further comprises a tilt detector for detecting a relative tilt of the objective lens and the optical disk in accordance with a luminous energy incident on the luminous flux incident area of the light sensing device.
A further aspect of the invention is directed to an integrated circuit for deriving a tilt signal corresponding to a relative tilt of an objective lens and an optical disk in accordance with a signal from a light sensing device of an optical head for concentrating a laser beam emitted from a light source onto information tracks of the optical disk by means of the objective lens and sensing a luminous flux reflected by the information tracks by means of the light sensing device, wherein:
the light sensing device includes a luminous flux incident area divided into a plurality of regions by a straight vertical dividing line passing an optical axis of the objective lens and parallel with a direction tangent to the information tracks, straight first and second horizontal dividing lines normal to the vertical dividing line and symmetrically arranged with respect to the optical axis, a first and a second partition lines symmetrically arranged between the two horizontal dividing  lines with respect to the vertical dividing line while being distanced from the vertical dividing line and the two horizontal dividing lines; each of the first and second partition lines includes a pair of horizontal lines parallel with the two horizontal dividing lines, and an inner line connecting ends of the horizontal lines toward the vertical dividing line; the luminous flux incident area further includes an N region crossing over the vertical dividing line between the two horizontal dividing lines, one region partitioned between the two horizontal dividing lines by the vertical dividing line being divided into an A2 region at a side toward the optical axis and a remaining A1 region by the first partition line, and the other region partitioned between the two horizontal dividing lines by the vertical dividing line being divided into a B2 region at a side toward the optical axis and a remaining B1 region by the second partition line; the A1 region and the B1 region are symmetrically arranged with respect to the vertical dividing line; the A2 region and the B2 region are symmetrically arranged with respect to the vertical dividing line; and a difference signal between a first difference signal, which is a difference signal between a signal obtained from the A1 region of the light sensing device and a signal obtained from the B1 region of the light sensing device, and a second difference signal, which is a difference signal between a signal obtained from the A2 region of the light sensing device and a signal  obtained from the B2 region of the light sensing device, is generated after multiplying at least one of the first and second difference signals by a specified weight coefficient, and the tilt signal is derived in accordance with the generated difference signal.
A further aspect of the invention is directed to an integrated circuit for deriving a tilt signal corresponding to a relative tilt of an objective lens and an optical disk in accordance with a signal from a light sensing device of an optical head for concentrating a laser beam emitted from a light source onto information tracks of the optical disk by means of the objective lens and sensing a luminous flux reflected by the information tracks by means of the light sensing device, wherein: the light sensing device includes a luminous flux incident area divided into a plurality of regions by a straight vertical dividing line passing an optical axis of the objective lens and parallel with a direction tangent to the information tracks, a first and a second partition lines symmetrically arranged with respect to the vertical dividing line while being distanced from the vertical dividing line, a third partition line arranged between the vertical dividing line and the first partition line, a fourth partition line symmetrically arranged with the third partition line with respect to the optical axis between the vertical dividing line and the second partition line; each of the first and second partition lines includes a  pair of horizontal lines normal to the vertical dividing line, and an inner line connecting ends of the horizontal lines toward the vertical dividing line; each of the third and fourth partition lines includes a pair of horizontal lines normal to the vertical dividing line, and an inner line connecting ends of the horizontal lines toward the vertical dividing line; the luminous flux incident area further includes an N region crossing over the vertical dividing line, a region at a side of the third partition line opposite from the optical axis being divided into an A5 region at a side toward the third partition line and a remaining A1 region by the first partition line, and a region at a side of the fourth partition line opposite from the optical axis being divided into a B5 region at a side toward the fourth partition line and a remaining B1 region by the second partition line; the A1 region and the B1 region are symmetrically arranged with respect to the vertical dividing line; the A5 region and the B5 region are symmetrically arranged with respect to the vertical dividing line; and a difference signal between a first difference signal, which is a difference signal between a signal obtained from the A1 region of the light sensing device and a signal obtained from the B1 region of the light sensing device, and a second difference signal, which is a difference signal between a signal obtained from the A5 region of the light sensing device and a signal obtained from the B5 region of the light sensing device, is generated after  multiplying at least one of the first and second difference signals by a specified weight coefficient, and the tilt signal is derived in accordance with the generated difference signal.
A further aspect of the invention is directed to an integrated circuit for deriving a tilt signal corresponding to a relative tilt of an objective lens and an optical disk in accordance with a signal from a light sensing device of an optical head for concentrating a laser beam emitted from a light source onto information tracks of the optical disk by means of the objective lens and sensing a luminous flux reflected by the information tracks by means of the light sensing device, wherein: the light sensing device includes a luminous flux incident area divided into a plurality of regions by a straight vertical dividing line passing an optical axis of the objective lens and parallel with a direction tangent to the information tracks, straight first and second horizontal dividing lines normal to the vertical dividing line and symmetrically arranged with respect to the optical axis, a third and a fourth horizontal dividing lines at a distance to and parallel with the horizontal dividing lines in one region partitioned by the vertical dividing line between the first and second horizontal dividing lines, a fifth and a sixth horizontal dividing lines at a distance to and parallel with the horizontal dividing lines in the other region partitioned by the vertical dividing line between the first and second horizontal dividing lines, a  seventh and an eighth horizontal dividing lines arranged at sides of the first and second horizontal dividing lines opposite from the optical axis, in parallel with the first and second horizontal dividing lines and symmetrically with respect to the optical axis; the luminous flux incident area further includes an N region crossing over the vertical dividing line between the first and second horizontal dividing lines; a region between the first and second horizontal dividing lines in the one region partitioned by the vertical dividing line is divided into an A1 region between the third and fourth horizontal dividing lines and remaining A7 regions; a region between the first and second horizontal dividing lines in the other region partitioned by the vertical dividing line is divided into a B1 region between the fifth and sixth horizontal dividing lines and remaining B7 regions; regions at sides of the seventh and eighth horizontal dividing lines opposite from the optical axis are defined as N2 regions; the A1 region and the B1 region are symmetrically arranged with respect to the vertical dividing line; the A7 region and the B7 region are symmetrically arranged with respect to the vertical dividing line; and a difference signal between a first difference signal, which is a difference signal between a signal obtained from the A1 region of the light sensing device and a signal obtained from the B1 region of the light sensing device, and a second difference signal, which is a difference signal between a signal obtained from the A7 region of the light  sensing device and a signal obtained from the B7 region of the light sensing device, is generated after multiplying at least one of the first and second difference signals by a specified weight coefficient, and the tilt signal is derived in accordance with the generated difference signal.
A further aspect of the invention is directed to an integrated circuit for deriving a tilt signal corresponding to a relative tilt of an objective lens and an optical disk in accordance with a signal from a light sensing device of an optical head for concentrating a laser beam emitted from a light source onto information tracks of the optical disk by means of the objective lens and sensing a luminous flux reflected by the information tracks by means of the light sensing device, wherein: the light sensing device includes a luminous flux incident area divided into a plurality of regions by a straight vertical dividing line passing an optical axis of the objective lens and parallel with a direction tangent to the information tracks: a pair of first horizontal dividing lines normal to the vertical dividing line and symmetrically arranged with respect to the vertical dividing line; a pair of second horizontal dividing lines parallel with the pair of first horizontal dividing lines and symmetrically arranged with respect to the vertical dividing line; a third and a fourth horizontal dividing lines at a distance to and parallel with the first and second horizontal dividing lines in one region partitioned by the  vertical dividing line between the first and second horizontal dividing lines; a fifth and a sixth horizontal dividing lines at a distance to and parallel with the first and second horizontal dividing lines in the other region partitioned by the vertical dividing line between the first and second horizontal dividing lines; a first inner line extending in parallel with the vertical dividing line and connecting inner ends of the first horizontal dividing line at one side, the third horizontal dividing line, the fourth horizontal dividing line and the second horizontal dividing line at one side; a second inner line extending in parallel with the vertical dividing line and connecting inner ends the first horizontal dividing line at the other side, the fifth horizontal dividing line, the sixth horizontal dividing line and the second horizontal dividing line at the other side; and seventh and eighth horizontal dividing lines arranged at sides of the first and second horizontal dividing lines opposite from the optical axis, in parallel with the first and second horizontal dividing lines and symmetrically with respect to the optical axis: the luminous flux incident area further includes an N region crossing over the vertical dividing line between the first and second inner lines: a region defined by the first horizontal dividing line, the second horizontal dividing line and the first inner line in the one region partitioned by the vertical dividing line is divided into an A1 region between the third and fourth horizontal dividing  lines and remaining A7 regions: a region defined by the first horizontal dividing line, the second horizontal dividing line and the second inner line in the other region partitioned by the vertical dividing line is divided into a B1 region between the fifth and sixth horizontal dividing lines and remaining B7 regions: regions at sides of the seventh and eighth horizontal dividing lines opposite from the optical axis are defined as N2 regions: the pair of first horizontal dividing lines and the pair of second horizontal dividing lines are symmetrically arranged with respect to the optical axis: the A1 region and the B1 region are symmetrically arranged with respect to the vertical dividing line: the A7 region and the B7 region are symmetrically arranged with respect to the vertical dividing line: and a difference signal between a first difference signal, which is a difference signal between a signal obtained from the A1 region of the light sensing device and a signal obtained from the B1 region of the light sensing device, and a second difference signal, which is a difference signal between a signal obtained from the A7 region of the light sensing device and a signal obtained from the B7 region of the light sensing device, is generated after multiplying at least one of the first and second difference signals by a specified weight coefficient, and the tilt signal is derived in accordance with the generated difference signal. 
According to the present invention, even in a place where reflectivity differs at adjacent information tracks, the tilt of an optical disk can be detected with high precision while being little influenced by the defocusing and a displacement of an objective lens.
These and other objects, features, aspects and advantages of the present invention will become more apparent upon a reading of the following detailed description and accompanying drawings.